Embodiments of the invention are directed, in general, to communication systems and, more specifically, to optimizing the spectral efficiency of the transmission of control channels in the downlink of a communication system.
The global market for both voice and data communication services continues to grow as does use of the systems which deliver such services. As communication systems evolve, system design has become increasingly demanding in relation to equipment and performance requirements. Future generations of communication systems, will be required to provide high quality high transmission rate data services in addition to high quality voice services. Orthogonal Frequency Division Multiplexing (OFDM) is a technique that will allow for high speed voice and data communication services.
Orthogonal Frequency Division Multiplexing (OFDM) is based on the well-known technique of Frequency Division Multiplexing (FDM). OFDM technique relies on the orthogonality properties of the fast Fourier transform (FFT) and the inverse fast Fourier transform (IFFT) to eliminate interference between carriers. At the transmitter, the precise setting of the carrier frequencies is performed by the IFFT. The data is encoded into constellation points by multiple (one for each carrier) constellation encoders. The complex values of the constellation encoder outputs are the inputs to the IFFT. For wireless transmission, the outputs of the IFFT are converted to an analog waveform, up-converted to a radio frequency, amplified, and transmitted. At the receiver, the reverse process is performed. The received signal (input signal) is amplified, down converted to a band suitable for analog to digital conversion, digitized, and processed by a FFT to recover the carriers. The multiple carriers are then demodulated in multiple constellation decoders (one for each carrier), recovering the original data. Since an IFFT is used to combine the carriers at the transmitter and a corresponding FFT is used to separate the carriers at the receiver, the process has potentially zero inter-carrier interference such as when the sub-carriers are separated in frequency by an amount larger than the maximum expected Doppler shift.
FIG. 1 is a diagram illustrative of the Frequency 103-Time 101 Representation 100 of an OFDM Signal. In FDM different streams of information are mapped onto separate parallel frequency channels 140. Each FDM channel is separated from the others by a frequency guard band to reduce interference between adjacent channels.
The OFDM technique differs from traditional FDM in the following interrelated ways:                1. multiple carriers (called sub-carriers 150) carry the information stream;        2. the sub-carriers 150 are orthogonal to each other; and        3. a Cyclic Prefix (CP) 110 (also known as guard interval) is added to each symbol 120 to combat the channel delay spread and avoid OFDM inter-symbol interference (ISI).        
The data/information carried by each sub-carrier 150 may be user data of many forms, including text, voice, video, and the like. In addition, the data includes control data, a particular type of which is discussed below. As a result of the orthogonality, ideally each receiving element tuned to a given sub-carrier does not perceive any of the signals communicated at any other of the sub-carriers. Given this aspect, various benefits arise. For example, OFDM is able to use orthogonal sub-carriers and, as a result, thorough use is made of the overall OFDM spectrum. As another example, in many wireless systems, the same transmitted signal arrives at the receiver at different times having traveled different lengths due to reflections in the channel between the transmitter and receiver. Each different arrival of the same originally-transmitted signal is typically referred to as a multi-path. Typically, multi-paths interfere with one another, which is sometimes referred to as InterSymbol Interference (ISI) because each path includes transmitted data referred to as symbols. Nonetheless, the orthogonality implemented by OFDM with a CP considerably reduces or eliminates ISI and, as a result, a less complex receiver structure, such as one without an equalizer (one-tap “equalizer” is used), may be implemented in an OFDM system.
The Cyclic Prefix (CP) (also referred to as guard interval) is added to each symbol to combat the channel delay spread and avoid ISI. FIG. 2 is a diagram illustrative of using CP to eliminate ISI and perform frequency domain equalization. Blocks 200 each comprising cyclic prefix (CP) 210 coupled to data symbols 220 to perform frequency domain equalization. OFDM typically allows the application of simple, 1-tap, frequency domain equalization (FDE) through the use of a CP 210 at every FFT processing block 200 to suppress multi-path interference. Two blocks are shown for drawing convenience. CP 210 eliminates inter-data-block interference and multi-access interference using Frequency Division Multiple Access (FDMA).
Since orthogonality is typically guaranteed between overlapping sub-carriers and between consecutive OFDM symbols in the presence of time/frequency dispersive channels, the data symbol density in the time-frequency plane can be maximized and high data rates can be very efficiently achieved for high Signal-to-Interference and Noise Ratios (SINR).
FIG. 3 is a diagram illustrative of CP Insertion. A number of samples are typically inserted between useful OFDM symbols 320 (guard interval) to combat OFDM ISI induced by channel dispersion, assist receiver synchronization, and aid spectral shaping. The guard interval 310 is typically a prefix that is inserted 350 at the beginning of the useful OFDM symbol (OFDM symbol without the CP) 320. The CP duration 315 should be sufficient to cover most of the delay-spread energy of a radio channel impulse response. It should also be as small as possible since it represents overhead and reduces OFDM efficiency. Prefix 310 is generated using a last block of samples 340 from the useful OFDM symbol 330 and is therefore a cyclic extension to the OFDM symbol (cyclic prefix).
When the channel delay spread exceeds the CP duration 315, the energy contained in the ISI should be much smaller than the useful OFDM symbol energy and therefore, the OFDM symbol duration 325 should be much larger than the channel delay spread. However, the OFDM symbol duration 325 should be smaller than the minimum channel coherence time in order to maintain the OFDM ability to combat fast temporal fading. Otherwise, the channel may not always be constant over the OFDM symbol and this may result in inter-sub-carrier orthogonality loss in fast fading channels. Since the channel coherence time is inversely proportional to the maximum Doppler shift (time-frequency duality), this implies that the symbol duration should be much smaller than the inverse of the maximum Doppler shift.
The large number of OFDM sub-carriers makes the bandwidth of individual sub-carriers small relative to the total signal bandwidth. With an adequate number of sub-carriers, the inter-carrier spacing is much narrower than the channel coherence bandwidth. Since the channel coherence bandwidth is inversely proportional to the channel delay spread, the sub-carrier separation is generally designed to be much smaller that the inverse of the channel coherence time. Then, the fading on each sub-carrier appears flat in frequency and this enables 1-tap frequency equalization, use of high order modulation, and effective utilization of multiple transmitter and receiver antenna techniques such as Multiple Input/Multiple Output (MIMO). Therefore, OFDM effectively converts a frequency-selective channel into a parallel collection of frequency flat sub-channels and enables a very simple receiver. Moreover, in order to combat Doppler effects, the inter-carrier spacing should be much larger than the maximum Doppler shift.
FIG. 4 shows the concepts of frequency diversity 400 and multi-user diversity 405. Using link adaptation techniques based on the estimated dynamic channel properties, the OFDM transmitter can adapt the transmitted signal to each User Equipment (UE) to match channel conditions and approach the ideal capacity of frequency-selective channel. Thanks to such properties as flattened channel per sub-carrier, high-order modulation, orthogonal sub-carriers, and MIMO, it is possible to improve spectrum utilization and increase achievable peak data rate in OFDM system. Also, OFDM can provide scalability for various channel bandwidths (i.e. 1.25, 2.5, 5, 10, 20 MHz) without significantly increasing complexity.
OFDM may be combined with Frequency Division Multiple Access (FDMA) in an Orthogonal Frequency Division Multiple Access (OFDMA) system to allow multiplexing of multiple UEs over the available bandwidth. Because OFDMA assigns UEs to isolated frequency sub-carriers, intra-cell interference may be avoided and high data rate may be achieved. The base station (or Node B) scheduler assigns physical channels based on Channel Quality Indication (CQI) feedback information from the UEs, thus effectively controlling the multiple-access mechanism in the cell. For example, in FIG. 4, transmission to each of the three UEs 401, 402, 403 is scheduled at frequency sub-bands where the channel frequency response allows for higher SINR relative to other sub-bands. This is represented by the Received signal levels R401, R402, and R403 for users 401, 402 and 403 at Frequencies F401, F402, and F403 respectively.
OFDM can use frequency-dependent scheduling with optimal per sub-band Modulation & Coding Scheme (MCS) selection. For each UE and each Transmission Time Interval (TTI), the Node B scheduler selects for transmission with the appropriate MCS a group of the active UEs in the cell, according to some criteria that typically incorporate the achievable SINR per sub-band based on the CQI feedback. A UE may be assigned the same sub-band for transmission or reception of its data signal during the entire TTI. In addition, sub-carriers or group of sub-carriers may be reserved to transmit pilot, control signaling or other channels. Multiplexing may also be performed in the time dimension, as long as it occurs at the OFDM symbol rate or at a multiple of the symbol rate (i.e. from one TTI to the next). The MCS used for each sub-carrier or group of sub-carriers can also be changed at the corresponding rate, keeping the computational simplicity of the FFT-based implementation. This allows 2-dimensional time-frequency multiplexing, as shown in FIG. 5 and FIG. 6.
Turning now to FIG. 5, which is a diagram illustrative of a configuration for multi-user diversity. The minimum frequency sub-band used for frequency-dependent scheduling of a UE typically comprises several sub-carriers and may be referred to as a Resource Block (RB) 520. Reference number 520 is only pointing to one of the 8 RBs per OFDM symbol shown as example and for drawing clarity. RB 520 is shown with RB bandwidth 525 (typically comprising of a predetermined number of sub-carriers) in frequency dimension and time duration 510 (typically comprising of a predetermined number of OFDM symbols such as one TTI) in time dimension. Each RB may be comprised of continuous sub-carriers and thus be localized in nature to afford frequency-dependent scheduling (localized scheduling). A high data rate UE may use several RBs within same TTI 530. UE #1 is shown as an example of a high rate UE. Low data rate UEs requiring few time-frequency resources may be multiplexed within the same RB 540 or, alternatively, the RB size may be selected to be small enough to accommodate the lowest expected data rate.
Alternatively referring to FIG. 6, which is a diagram illustrative of a configuration for frequency diversity, an RB 620 may correspond to a number of sub-carriers substantially occupying the entire bandwidth thereby offering frequency diversity (distributed scheduling). This may be useful in situations where CQI feedback per RB is not available or it is unreliable (as is the case for high speed UEs) and only CQI over the entire frequency band is meaningful. Therefore, a sub-band (or RB) consists of a set of sub-carriers that may be either consecutive or dispersed over the entire spectrum. It should be noted, that another option to achieve frequency diversity is to assign to a UE two or more RBs with each RB comprising of contiguous sub-carriers but and with each RB occupying non-contiguous parts of the bandwidth. In such cases, an RB always consists of a contiguous set of sub-carriers (for both localized and distributed scheduling).
By assigning transmission to various simultaneously scheduled UEs in different RBs, the Node B scheduler can provide intra-cell orthogonality among the various transmitted signals. Moreover, for each individual signal, the presence of the cyclic prefix provides protection from multipath propagation and maintains in this manner the signal orthogonality.
Each scheduled UE is informed of its scheduling assignment by the serving Node B through the downlink (DL) control channel. This control channel typically carries the scheduled UE identities (IDs), RB assignment information, the MCS used to transmit the data, the transport block size, and hybrid ARQ (HARQ) information relating to possible data packet re-transmissions in case of a previous erroneous reception for the same data packet. The control channel may also optionally carry additional information such as for the support of a multi-input multi-output (MIMO) scheme for transmission and reception with multiple antennas. A scheduling assignment may relate either to data transmission from the Node B to a UE (DL of a communication system) or to data transmission from a UE to the Node B (UL of a communication system).
According to one prior art method for the control channel transmission, such as the one employed by the WiMax communication system, the control channel information for all scheduled UEs is jointly coded with a known MCS. This MCS has to be a low one in terms of spectral efficiency (for example, QPSK modulation and low rate convolutional or turbo coding with possible repetitions) as the control channel needs to be received by all UEs in the serving Node B area including ones potentially experiencing very low SINR. As a result, the control channel size and corresponding overhead may become excessively large, thereby adversely affecting the system throughput.
According to another prior art method for the control channel transmission, the control channel information for all scheduled UEs is separately coded with a known fixed MCS and power control may be applied to the transmission. In this manner, the control channel transmission power for UEs located closer to the serving Node B is reduced while the transmission power for UEs located near the edge is increased to account for the path loss. This scheme improves the overall spectral efficiency by reducing the interference caused by the control channel transmission. Nevertheless, as power control adaptation is based on prior CQI feedback from UEs, it is not generally possible to predict the future interference conditions in order for the power control to be effective. Moreover, this transmission method may result to significant and unpredictable interference variations making the whole scheduling process less reliable. For example, a conflict occurs when the control channels to cell edge UEs in adjacent Node Bs are transmitted from these Node Bs using substantially the same frequency resources. Then, transmission power control is ineffective as it is substantially cancelled since the interfering transmissions apply the same transmission power control. Separate transmission of the control channel to scheduled UEs with a fixed MCS is used in the 3GPP HSDPA communication system.
Thus, there is a need for a method to provide reliable transmission of the DL control channel while optimizing the corresponding spectral efficiency of the transmission in a communication system.